Short Film
The Double Helix
Educator Materials
IN-DEPTHFILMGUIDE
DESCRIPTION
ThefilmTheDoubleHelixdescribesthetrailofevidenceJamesWatsonandFrancisCrickfollowedtodiscover
thedouble-helicalstructureofDNA.Theirmodel’sbeautifulandsimplestructureimmediatelyrevealedhow
geneticinformationisstoredandpassedfromonegenerationtothenext.
KEYCONCEPTS
A. DNAisapolymerofnucleotidemonomers,eachconsistingofaphosphate,adeoxyribosesugar,andoneof
fournitrogenousbases:adenine(A),thymine(T),guanine(G),orcytosine(C).
B. TherelativeamountsofA,T,G,andCbasesvaryfromonespeciestoanother;however,intheDNAofany
cellfromorganismswithinasinglespecies,theamountofAisequaltotheamountofTandtheamountof
GisequaltotheamountofC.ThisfindingcanbeexplainedbythefactthatintheDNAdoublehelix,Apairs
withTandGwithC.
C. EvenbeforethestructureofDNAwassolved,studiesindicatedthatthegeneticmaterialmustbeableto
storeinformation;befaithfullyreplicatedandbepassedonfromgenerationtogeneration;andallowfor
changes,andthusevolution,tooccur.ThestructureofthedoubleheliximmediatelyshowedthatDNAhad
theseproperties.
D. Scientistsusedifferenttechniquestomeasurethingsthataretoolargeortoosmalltosee.Thestructureof
DNAwasdeterminedbycombiningmathematicalinterpretationsofx-raycrystallographydataandchemical
data.
E. Scientistsbuildmodelsbasedonwhattheyknowfrompreviousresearchtoderivetestablehypotheses.
Datafromexperimentsareusedtorevisemodelsandaskadditionalresearchquestions.Theultimategoalis
tofindamodelthatisvalidinallormostoftheobservations.
F. Theprocessofscientificdiscoveryinvolvesbrainstormingandevaluatingideas,makingmistakes,and
rethinkingthoseideasbasedonevidence.Failureisanimportantaspectofscientificdiscovery.
G. Communicationamongscientistsplaysacrucialroleinscientificdiscoveries.TounlockthestructureofDNA,
WatsonandCrickalsoreliedonobservationsmadebyotherscientists.
CURRICULUMANDTEXTBOOKCONNECTIONS
Curriculum
NGSS(April2013)
AP(2012–13)
IB(2016)
Standards
MS-LS3.A,MS-LS3.B,MS-ETS1.B,MS-PS1.B
HS-PS2.B,HS-PS4.C,HS-LS1.A,HS.LS3.A,HS.LS3.B
3.A.1,4.A.1
2.6,2.7,7.1
Textbook
MillerandLevine,Biology(2010ed.)
Reeseetal.,CampbellBiology(9thed.)
ChapterSections
12.1,12.2,12.3
5.5,16.1,16.3
www.BioInteractive.org
RevisedMarch2017
Page1of17
Short Film
The Double Helix
Educator Materials
KEYTERMS
chemicalbonds,chromosomes,DNA,genes,helix,hydrogenbonds,mutation,nucleotides,nucleus,proteins,
structure,replication
PRIORKNOWLEDGE
Studentsshould
•
•
•
•
•
knowthatbiologicalmoleculesarecomposedofdifferenttypesofatoms,includingcarbon,oxygen,
nitrogen,andhydrogenatoms;
knowthattheshapesofbiologicalmoleculesdependonthearrangementofthecomponentatomsand
theirchemicalbonds,whichconstrainthedistancesbetweenatoms;
knowthatgenesaremadeofDNA,thattheyareinheritedfromonegenerationtothenext,andthat
mutationsarechangesintheDNAsequence;
haveabasicunderstandingofDNAreplicationandthecentraldogmathatDNAistranscribedtoRNAand
RNAistranslatedintoproteins;and
befamiliarwiththescientificprocessoftestingideaswithevidence.
PAUSEPOINTS
Thefilmmaybeviewedinitsentiretyorpausedatspecificpointstoreviewcontentwithstudents.Thetable
belowlistssuggestedpausepoints,indicatingthebeginningandendtimesinminutesinthefilm.
Begin
1 0:00
End
4:08
2 4:09
9:05
www.BioInteractive.org
ContentDescription
• Intheearly20thcenturyseveralscientists
weretryingtosolvethemysteryofthe
structureofDNAinordertobetter
understandinheritanceoftraits.
• JamesWatson&FrancesCrickmetin
Cambridgein1951.Bothwereinterestedin
findingthestructureofthegene.
• Inthe1920s,geneshadbeenlocatedinthe
nucleusandassociatedwithchromosomes.
Scientistsneededtodetermineifgeneswere
inproteinsorDNA.
• DNAisapolymerofnucleotidemonomers,
eachconsistingofaphosphate,a
deoxyribosesugar,andoneoffour
nitrogenousbases:adenine(A),thymine(T),
guanine(G),orcytosine(C).
• OswoldAverydemonstratedthatDNAcould
carrygeneticinformation.
• X-raycrystallographyisatechniquefor
determiningmolecularstructure.Itcan
determinethelocationofatomswithina
ReviewQuestions
• Whatare
chromosomes
madeof?
• Wherearegenes
found?
• Whatisthe
structureofDNA?
• WhywasOswold
Avery’swork
significantto
WatsonandCrick?
Standards
NGSS(April
2013)
MS-LS3.A,
MS-LS3.B,
MS-ETS1.B,
MS-PS1.A,
HS-LS1.A,HSLS3.A,HSLS3.B
APBiology
(2013)
3.A.1,4.A.1
IBBiology
(2016)
2.6,2.7,7.1
•
NGSS(April
2013)
MS-LS3.A,
Whywasx-ray
crystallography
beingusedto
RevisedMarch2017
Page2of17
Short Film
The Double Helix
Educator Materials
•
•
•
•
3 9:06
13:50
•
•
•
•
4 13:51
16:53
•
•
•
www.BioInteractive.org
molecule.
MauriceWilkins,aphysicistatKingsCollege,
wasusingX-raycrystallographytodetermine
thestructureofDNA.
RosalindFranklinwasacolleagueofWilkins,
butsheandWilkinsworkedseparately.
LinusPaulingwasalsosearchingforDNA’s
structure.
Pauling,WatsonandCrickbelievedDNAwas
ahelicalmolecule.WatsonandCrick’sfirst
modelofDNAwasincorrect.Theyreliedon
informationfromotherscientists.
ThestructureofDNAwasdeterminedby
•
combiningmathematicalinterpretationsofxraycrystallographydataandchemicaldata. •
WilkinsshowedWatsonanx-ray
crystallographypicturetakenbyFranklin
(Photo51).TheX-shapeddiffractionpattern
ischaracteristicofahelicalmolecule.
Chargaffhadreportedthatbaseratioswere
alwaysthesameinallorganisms.
WatsonandCrickbuiltamodelofDNAasa
doublehelix,withthebasesarrangedinside.
Solvingthestructurehadfar-reaching
implicationsforbiology.
Thecomplementarynatureofthebases(A-T
andG-C)providedamethodforreplicatinga
newcomplementarycopy.
DNA’sstructurerevealedhowgenetic
informationisstoredinthesequenceofthe
basesandhowmutationscanhappen.
•
•
determinethe
structureofDNA?
MS-ETS1.B,
MS-PS1.A,
HS-PS4.C,HSLS1.A,HSLS3.A,
APBiology
(2013)
3.A.1,4.A.1
IBBiology
(2016)
2.6,7.1
NGSS(April
Whatisthe
structureofDNA? 2013)
Whatwerethekey MS-LS3.A,
piecesofevidence MS-ETS1.B,
MS-PS1.A,
thatledWatson
HS-PS2.B,HSandCrickto
PS4.C,HSdeterminethat
LS1.A,HSstructure?
LS3.A
APBiology
(2013)
3.A.1,4.A.1
IBBiology
(2016)
2.6,7.1
NGSS(April
HowdoesDNA’s
2013)
structureexplain
MS-LS3.A,
thestabilityof
MS-LS3.B,
life?
MS-PS1.A,
Howdoesit
HS-PS2.B,HSexplainthe
mutabilityoflife? LS1.A,HSLS3.A,HSLS3.B
APBiology
(2013)
3.A.1,4.A.1
IBBiology
(2016)
2.6,2.7,7.1
RevisedMarch2017
Page3of17
Short Film
The Double Helix
Educator Materials
BACKGROUND
Thediscoveryofthethree-dimensionalstructureofDNAwasmadepossiblebyearlierworkofmanyscientists
whohaduncoveredevidenceaboutheredity,genes,andDNA.ThefilmTheDoubleHelixmentionsmanyof
thesefindings,whicharelistedinmoredetailbelow.
Thetrailofevidencebeginsinthe19thcentury,whenAustrianmonkGregorMendeldiscoveredpatternsinthe
waycharacteristics,ortraits,areinheritedfromonegenerationtothenext.Doingexperimentsusinggarden
peaplants,Mendelfoundthattraitslikepeashapeandcolorarepassedfromparenttooffspringasdiscrete
unitsinapredictableway.
Intheearly1900s,AmericangeneticistThomasHuntMorgandemonstratedthatthesediscreteunitsof
heredity—orgenesastheywerebynowcalled—werelocatedonchromosomes.Chromosomeswereknownto
becomposedofDNAandprotein,butitwasunclearwhichofthetwotypesofmoleculeswasthesourceof
geneticinformation.
Mostresearchersfavoredproteinasthegeneticmaterial.Proteinsarebuiltfrom20distinctaminoacid
componentsandshowgreatstructuraldiversityandspecificity.Incomparison,DNAseemedmonotonous.
FrederichMiescher,aSwissphysician,hadfirstisolatedDNAfromwhitebloodcellsin1871.Shortlyafterthat,
AmericanbiochemistPhoebusLeveneidentifiedthecomponentsofDNA:deoxyribosesugar,phosphate,and
oneoffourdifferentnitrogenousbases.
ThenucleotidestructureofDNA.
DNAconsistsofchainsof
nucleotides,whicharemadeofa
sugar,phosphate,andoneoffour
bases.DNAseemedlikeaboring
substancecomparedtoproteins.
In1938,BritishphysicistWilliamAstburytookthefirstx-raydiffractionimagesofDNA.Heusedtheseimagesto
buildamodelofthestructureofDNAusingmetalplatesandrods.Whilehismodelwasverytentativeand
containederrors,Astburycorrectlypositionedthebaseslyingflat,stackedlikeapileofpennies,0.34nmapart.
AseriesofexperimentssetthestageforestablishingthatgenesweremadeofDNAandnotproteins.Frederick
Griffith’s1928experimentsshowedthatpneumococcalbacteriacouldtransfergeneticinformationbetween
differentstrainsthroughaprocesshecalledtransformation.OswaldAvery,ColinMacLeod,andMaclyn
McCartydeterminedthatthemoleculeresponsibleforthistransformationwasDNAandnotprotein.Averyand
colleagues’1944paperwasinitiallymetwithskepticism,asmanyscientistscontinuedtobelievethatproteins
werethegeneticmaterial.
Inthemeantime,moreinformationwasemergingaboutthestructureofDNA.AmericanbiochemistErwin
Chargaffreportedin1949thattheproportionsofthefournucleotidesinaDNAmoleculevariedbetween
www.BioInteractive.org
RevisedMarch2017
Page4of17
Short Film
The Double Helix
Educator Materials
species.However,withinaspecies,thepercentagesofadenine(A)andthymine(T)baseswerealwaysequal,as
werethepercentagesofguanine(G)andcytosine(C).Thesignificanceofthisfindingwasnotappreciateduntil
WatsonhadtheinsighttouseittoinformthemodelofDNAhewasbuilding.
RelativeProportions(%)ofBasesinDNA
Organism
A
T
G
Human
30.9 29.4 19.9 19.8
Chicken
28.8 29.2 20.5 21.5
C
Grasshopper 29.3 29.3 20.5 20.7
SeaUrchin
32.8 32.1 17.7 17.3
E.coli
24.7 23.6 26.0 25.7
Chargaff’srule.ErwinChargaffdiscoveredthatinaDNAmolecule,theproportionofadenine(A)always
equalsthatofthymine(T)andtheproportionofguanine(G)alwaysequalsthatofcytosine(C).
ThemostconvincingevidencethatDNAwasthemoleculeofhereditycamefromAlfredHersheyandMartha
Chaseinapaperpublishedin1952.WorkingatColdSpringHarborLaboratoryonLongIsland,NY,theyused
radioactiveisotopesofsulfurandpotassiumtolabelproteinsandDNA,respectively,inbacteriophages—which
arevirusesthatinfectbacteria.TheexperimentshowedthatthebacteriophageDNA,andnottheproteins,
enteredbacteriaforinfection.
AtthetimeoftheHershey-Chaseexperiment,anumberofgroupshadstartedworkingtodeterminethe
molecularstructureofDNA.AmongthemwasLinusPaulingofCalTech,famousforhavingsolvedthestructure
ofseveralproteinsbybuildingmodelsbasedonchemicalbondingprinciplesandbiochemicalevidence.Hewas
aninspirationtoWatsonandCrickaswellasthepersonmostlikelytosolvethestructurebeforethem.In1951,
Paulinghadproposedthatthepolypeptidechainsofproteinsfoldinα-helicalstructures.Today,theα-helixis
knowntoformthebackboneoftensofthousandsofproteins.
Paulinghadturnedhismodel-buildingskillstothestructureofDNA.InEngland,MauriceWilkinsandRosalind
FranklinatKing’sCollegeLondonwereusingx-raycrystallographytoanalyzeDNA’sstructure.Despiteafew
confusingblurryspots,theinitialimagestheyobtainedhintedthatDNAmightcomeintheformofatwisted
spiral—orhelix.However,itwasnotclearhowthephosphates,sugars,andbaseswerearrayedwithinthathelix.
ShortlyafterWilkinsandFranklinbegantheirexperiments,WatsonandCrickdecidedtoworkonDNAaswell.
InspiredbyPauling’swork,theystartedbuildingmodelsofDNAmolecules.UnlikeWilkinsandFranklin’s,their
approachwastoformulateapossiblestructureofDNAandthendeterminewhetheritfitexperimental
observations.Inoneoftheirfirstattempts,theycreatedahelixwiththreesugar-phosphatechainsheldtogether
bychemicalbondsfacilitatedbymagnesiumions,withthebasesprojectingoutwardfromthiscentralbackbone.
www.BioInteractive.org
RevisedMarch2017
Page5of17
Short Film
The Double Helix
Educator Materials
Franklinsawthatthemodeldidnotfitthex-rayevidence.Basedonhermeasurements,DNAfiberscontainedat
least10timesasmuchwaterasWatsonandCrick’smodelallowedfor,andtherewasnoevidencethatDNAwas
associatedwithmagnesiumions.Asconstructed,themodelcouldnotexplainhowthethreephosphate
backbonescouldbeheldtogetheratthecenterofthemolecule.(ShortlybeforeWatsonandCrickproduced
theirsuccessfuldouble-helixmodel,Paulingproducedhisownflawedtriple-helixmodel,againwiththe
phosphatesinthecenterofthemolecule,butwithoutmagnesiumions.Paulinghadhydrogenbondsholdingthe
phosphatechainstogether,butbasedonwhatwasknownaboutchemicalbondsthatmodelwasnotvalid.)
Franklin’smeasurementsofthewaterassociatedwithDNAsuggestedthatthephosphategroupswouldbe
locatedinanaqueousenvironmentontheexterioroftheDNAmolecule.Ever-betterx-rayimages,including
Franklin’sfamousphotoB51,providedinformationaboutthedimensionsoftherepeatingsubunitsinaDNA
molecule.Inaddition,herimagesindicatedthatDNAmoleculeslookthesamewhentheyareturnedupside
downandfronttoback.WhenCrickfoundoutaboutDNA’sdyadsymmetry,heinferredthatthephosphate
chainsmustruninoppositeorientations,orantiparallel,tooneanother—abrilliantinsightthatFranklinand
othershadmissed.
B
A
C
DNAmoleculeshavedyad
symmetry.Onewaytoexplain
dyadsymmetryistopretend
thatthetwopencilsin(A)area
DNAcrystal.Whenflipped
upsidedown(B)andfrontto
back(C),itlooksthesameas
theoriginal(A).
BuildingonthesecluesandWilkinsandFranklin’smeasurements,WatsonandCrickonceagainturnedto
modelstotesttheirhypothesesofDNAstructure.Thistimetheytriedbuildingmodelswithtwoantiparallel
phosphatechainsontheoutsideofthemolecule.Inthisarrangement,thechainswouldhavetobeheld
togetherbythebasesontheinside,butWatsonandCrickwerenotsurehowthesebasesmightpairup.That’s
whentheyrememberedChargaff’sratios.CrickreasonedthatAmustalwayspairwithTandGwithC.Butwhere
werethebondsbetweenthesebases?TheyconsultedJ.N.Davidson’sTheBiochemistryofNucleicAcids,
publishedin1950.However,aswithotherbooksofthattime,itcontainedincorrectforms,ortautomers,of
guanineandthymine(seefigure).Nomatterhowtheytried,thebasesdidnotformanicehydrogen-bonding
pattern,asforexampleintheproteinbackboneofanα-helix.AvisitingAmericanchemist,JerryDonohue,
lookedattheirbasestructuresandrealizedthatthesewerewrong.
www.BioInteractive.org
RevisedMarch2017
Page6of17
Short Film
The Double Helix
Educator Materials
Thetextbookswerewrong.Guanineandthymine
canhavealternatemolecularstructuresbasedon
differentlocationsofaparticularhydrogenatom.
Thesetwotypesofstructuresareknownas
tautomers.Thetautomericformsofeachbase
existinequilibrium,butoneformismorestable
andthereforepredominatesundertheconditions
foundinsidemostcells.Intheearly1950s,
chemistrytextbookshaddrawingsofthe“wrong”
tautomersofguanineandthymine.Jerry
DonohuetoldJamesWatsonwhatthecorrect
structureswere.Thispieceofthepuzzleallowed
WatsontobuildpairsofA’sandT’sandofG’sand
C’swithaccuratehydrogenbonds.
OnceWatsonincorporatedthenew,correctshapesofthebasesintohismodel,hesawwherethehydrogen
bondswouldformandhismodelofDNAimmediatelyfellintoplace!
TheA-TandG-Cpairingswereconsistentwiththemeasurementsofthemoleculefromx-rayimages.The
hydrogenbondsbetweenthebasepairsmakethemoleculestructurallystable.Todayweknowthatthereare
twohydrogenbondsinanA-TpairandthreehydrogenbondsinaG-Cpair.However,atthetimeWatsonand
CrickbuilttheirmodeltheyhadinitiallyidentifiedtwohydrogenbondsbetweenbothA-TpairingsandG-C
pairings.
Basepairingsusedtoconstruct
thedoublehelix.Watsonand
Crickrejectedtheideaofathird
hydrogenbond(shownbythe
dottedlines)betweenguanine
andcytosinebecausedatahinted
thatsuchabondwouldbeweak.
Laterevidenceshowedthatthere
areinfactthreestronghydrogen
bondsinaguanine-cytosinepair.
(Thethirdbondisshowninblue.)
www.BioInteractive.org
RevisedMarch2017
Page7of17
Short Film
The Double Helix
Educator Materials
KeyEvidenceCamefromX-rayDiffraction
Developedearlyinthe20thcentury,x-raycrystallographyallowstheindirectobservationofmolecularstructures
toosmalltobeseenorphotographed.Thefather-sonteamofWilliamH.andWilliamL.BraggsharedtheNobel
Prizein1915forusingx-raystorevealhowtherepeatingstructuresofcrystalsform.Tablesaltwasthefirst
crystalstructuresolvedbyx-raycrystallographyin1914,soonfollowedbytherepeatingcarbonstructureof
diamond.DorothyHodgkinandMaxPerutzwerepioneersinsolvingthestructuresoforganicmolecules
containingmorecomplexatomicarrangements,includingcholesterol,penicillin,vitaminB12,insulin,and
hemoglobin.
WilliamL.BraggwasdirectoroftheCavendishLaboratoryatthetimethatWatsonandCrickwerethere.Hehad
beenstrivingtodeterminethestructuresofproteincomponents,butLinusPaulinggottherefirstbydiscovering
thestructureoftheα-helix.
X-raycrystallographyinvolvesmountingamoleculeonastageandbombardingitwithabeamofx-rays.The
wavelengthsofx-raysaresoshortthattheybounceoffatomswithinthemolecule,scatteringatspecificangles
thatdependonthedistancesbetweenatomsofvarioussizes.Thescatteredx-raysproducepatternsthatcanbe
capturedonphotographicfilmordigitally.Tointerpretthesepatterns,crystallographersmustdeterminewhen
x-raysscatteredfromdifferentatomsoverlap.Thisoverlapchangestheintensityofspotsinthex-raypattern.
Two-dimensionalimagestakenatdifferentanglesareconvertedintoathree-dimensionalmodelofthemolecule
usingmathematicalcalculationstermedFouriertransformations,whichallowthepositionsofatomswithinthe
moleculetobedetermined.Whenaveragedovermanyobservations,thesemeasurementscanbeaccurateto
justafractionofanångstrom(orone10-billionthofameter).
Thenumberofx-raysdiffractedbyasinglemoleculewouldsimplybetoosmalltobeobserved.Therefore,x-ray
crystallographyexaminesmanyidenticalmoleculespackedintoahighlyregularthree-dimensionalarrayrather
thansinglemolecules.Becausethex-raystravelthroughmanylayersofatoms,it’simportantthattheatoms
alwaysoccurinthesamearrangementinallthemoleculesinasample.Iftheydon’t,thex-raysarebentinto
overlappingpatterns,resultinginblurryorcompletelysmeareddiffractionpatternsthatcannotbeinterpreted
orprovideverypoorresolution.Becausecrystalshavearepeatingarrangementofatomsinidenticalorientation,
theyleaveapatternofsharp,clearspots.Forthisreason,biologicalmoleculesaretypicallyturnedintocrystals,
orcrystallized,beforetheyareanalyzedwithx-rays.Infactmuchtimeandeffortmaybespentin“growing”
crystalsofaparticularmoleculeofinterest,andthiscanbetherate-limitingstepinaresearchefforttosolvea
molecularstructure.
Franklintookx-raysnotofcrystalsbutofDNAfibersthat,althoughthin,consistedofthousandsofindividual
DNAmoleculestightlyarrayed,somewhatliketheindividualstrandsofthickrope,orhairgatheredintoapony
tail.Whilenotincrystalform,theDNAhadasufficientlyregulararrangementtodiffractx-raysinawaythat
produceddecipherablepatterns.
ByexposingDNAtovaryinglevelsofatmospherichumidity,FranklinandhergraduatestudentRaymondGosling
demonstratedthatDNAexistedintwoforms,whichtheycalledAandB.The“dry”Aformoccurswhenthe
relativehumidityislessthan75%andproducesascatteredx-raydiffractionpatternconsistingofmanydistinct
spots.Watermoleculesclingtothe“wet”B-formDNA,causingthestrandstoelongate,producinganX-shaped
diffractionpattern.Becauseallmoleculesincellsareimmersedinliquid,theB-formDNAistheformthatexists
primarilyinsidecells.Itwasnotuntil1980thatanactualcrystalstructureofmorethanacompleteturnofB
www.BioInteractive.org
RevisedMarch2017
Page8of17
Short Film
The Double Helix
Educator Materials
DNA,inwhichindividualatomsoftheDNAcouldbedistinguished,waspublished(Wing,R.etal.1980.Nature
287:755-8).
TheAandBformsofDNA.The“dry”AformofDNAandthe“wet”BformofDNAhavedistinct
structuresasshownbytheirx-raydiffractionpatternsontheleft.
FranklinhadinitiallyfocusedherattentiononA-formDNAbecauseshethoughtthoseimagescontainedmore
information.ItwasinfactoneoftheA-formphotosthathadrevealedthatthetwostrandsofDNAranin
oppositedirections,althoughneitherFranklinnortheothershadbeenabletointerprettheevidencetomake
thisconclusion—itwasFrancisCrickwhorealizeditssignificance.
ThefamousphotoB51,takenbyFranklinandGoslinginMay1952,wasofB-formDNA.(Asitwasthe51stphoto
taken,FranklinlabeledtheimageB51.)TheX-shapeddiffractionpatternischaracteristicofahelicalmolecule.
Independentlinesofevidencehaveconfirmedthatthediamondshapesformedbythearmsandlegsofthe“x”
indicatetherepetitionofthehelicalpatternaswellastheplacementofthephosphatesugarbackboneonthe
exteriorofthemoleculeandthebasesintheinterior.Analysisoftheblurrysmearscomposingthe“x”ofthe
photoallowsthecalculationofthedimensionsoftheDNAmolecule:aradiusof1.0nm,0.34nmbetweenbase
pairs,and3.4nm(10basepairs)withinacompleteturnofthehelix.AlthoughthemodelbuiltbyWatsonand
Crickwasbasedonfiberdiffraction,theirknowledgeofthechemicalnatureofthecomponentsofDNAallowed
themtobuildamodelthatincludedthepositionsofatomsandthechemicalbondsbetweenthem,whichwas
laterconfirmedbyhigh-resolutionx-raycrystallography.
www.BioInteractive.org
RevisedMarch2017
Page9of17
Short Film
The Double Helix
Educator Materials
PhotoB51.PhotoB51was
takenbyFranklinandGosling.
ItrevealedthatB-formDNA
wasadoublehelixwith10
nucleotidebasepairswithina
completeturnofthehelix.The
crossindicatesahelix.Thedark
patchesindicatethebases.
TheDiscoveryoftheStructureofDNALedtoKeyInsights
OnthemorningofFebruary28,1953,WatsonconstructedthemodelofDNAthatunlockedthesecretoflife.
Twomonthslater,WatsonandCrickpublishedtheirfindingsinNatureinapapertitled“Astructurefor
deoxyribosenucleicacid.”Inthesameissue,immediatelyfollowingWatsonandCrick’spaper,wasapaper
writtenbyWilkinsandhiscolleaguesAlecStokesandHerbertWilson.Thethirdpaperintheserieswaswritten
byFranklinandGosling.Wilkins’andFranklin’spaperspresentedtheevidencetheyhadobtained,corroborating
WatsonandCrick’sproposedstructureofDNA.
AnunderstandingofthestructureofDNAprovidedanexplanationofhowDNAfunctionedasthehereditary
material.WatsonandCricknotedthisintheirNaturepaper.Theywrote:“Ithasnotescapedournoticethatthe
specificpairingwehavepostulatedimmediatelysuggestsapossiblecopyingmechanismforthegenetic
material.”BecauseAisalwayspairedwithTandGwithC,theorderofbasesononestranddeterminesthe
orderontheother.Thus,ifaDNAmoleculewereunwound,eachstrandcouldbecopiedintoacomplementary
strand,producinganexactreplicaoftheoriginalmolecule.Errorsinthecopyingmechanismcouldresultin
mutations,orchangesintheDNAsequence,thatcouldbeinheritedbyfuturegenerations.
In1962,Watson,Crick,andWilkinswereawardedtheNobelPrizeinPhysiologyorMedicine“fortheir
discoveriesconcerningthemolecularstructureofnucleicacidsanditssignificanceforinformationtransferin
livingmaterial.”Franklin’sdeathin1958fromovariancancerprecludedherfromreceivingmanyofthehonors
forthediscoveryofDNA’sstructure,includingthepossibilityofsharingintheNobelPrize,whichcannotbe
awardedposthumously.
Ittookseveralyearsofsubsequentstudy,includingaclassic1958experimentbyAmericangeneticistsMatthew
MeselsonandFranklinStahl,beforetheexactrelationshipbetweenDNAstructureandreplicationwas
understood(Meselson,M.,andStahl,F.1958.ThereplicationofDNAinEscherichiacoli.Proceedingsofthe
NationalAcademyofSciences44,671–682).
DISCUSSIONPOINTS
•
Thewaythestoryunfoldsinthisshortfilmmaygivestudentstheimpressionthateachpieceofevidencefell
intoplaceoneaftertheother,inasomewhatlinearpath.Inrealitymostofthepieceswereinplayand
movingaroundinWatsonandCrick’sheadsandintheirdiscussionsastheyalmostcontinuouslythought
abouttheproblemofDNAovermanymonths.Theprocesswasoneoftrialanderror,andcirclingback
www.BioInteractive.org
RevisedMarch2017
Page10of17
Short Film
The Double Helix
Educator Materials
•
•
•
•
•
repeatedly.AfterseeingphotoB51,WatsonsuspectedthatDNAmightbeadoublehelix,buthecontinued
toconsiderbothadoubleortriplehelixuntilhebuiltadouble-helicalstructurethatfitalltheknown
evidence.Similarly,Franklin’sx-raydatahadsuggestedthatthephosphatechainswereontheoutside,but
thatpieceofevidencedidnotfituntilWatsonandCrickfiguredouthowthenitrogenousbasesmightpair
togetheratthecenter.
Atthebeginningofthefilm,SeanCarrollsetsthestagefortheunfoldingofthestorybysaying,“Thethreedimensionalarrangementofatomsinthosemoleculeshadtoexplainthestabilityoflife,sothattraitswere
passedfaithfullyfromgenerationtogeneration,andalsothemutabilityoflife.”Thenotionthatheredity
couldbeexplainedbythe“arrangementofatoms”ofmoleculeswasfirstraisedbyworld-renowned
physicistErwinSchrödinger.In1944,SchrödingerpublishedthebookWhatIsLife?inwhichhearguedthat
livingthingsshouldbeconsideredintermsofmolecularandatomicstructure,astheyobeythesamelawsof
chemistryandphysics.AccordingtoSchrödinger,genesarepassedfromgenerationtogenerationbecause
thegeneticcodewasaresultofthearrangementofatomswithinamolecule.Thesethoughtsinspireda
wholegenerationofresearchers,includingWatsonandCrick.Beforeyourstudentswatchthefilm,youmay
wanttodiscusswiththemtheimportanceofunderstandingthree-dimensionalmolecularstructures.Ask
them“Howdoesknowingthestructureofanyobjecttellyouaboutitsfunction?”
Therearemanyopportunitiesthroughoutthefilmtodiscussthenatureofscientificinquiry.Forexample,
sciencetriestoanswerquestionsaboutthenaturalworld.Intheearly1950s,theprinciplesofgeneticswere
known,butnooneknewwhatthegeneticmaterialwas,letalonehowanyphysicalorchemicalstructure
relatedtotheconsistencyofinheritanceandalsothecapacitytoevolvenewtraits.Leadstudentsina
discussionofthequestionsscientistsweretryingtoanswerbydeterminingthestructureofDNA.
Individualscientiststakedifferentapproachesintryingtounderstandaprocessorsolveaproblem,duein
parttodifferencesintraining,thetoolsavailablewheretheywork,andtheirpersonalities.Franklinand
WilkinswantedtosolvethestructureofDNAbyobtainingx-raydiffractiondata;theyreliedonexperiments
andobservations.WatsonandCrickbuilttheoreticalmodelswhichallowedthemtoseewhetherthose
modelsagreedwithwhatwasknownaboutchemicalbondingandx-raydata.Manyscientistsusea
combinationofdata-gatheringandhypothesis-testingapproaches.
Hypothesesmustbetestedandevaluatedagainstevidence.EarlierresearchersgatheredevidencethatDNA
wasthegeneticmaterial.Evidencefromx-raydiffractionpatternsandChargaff’sbase-pairingratios
supportedWatsonandCrick’sball-and-stickmodelofthestructureofDNA.Askstudentstoidentifythekey
piecesofevidencethatWatsonandCrickusedtoconstructtheirmodelofDNA.Someofthekeyevidence
presentedinthefilmincludesthestructureofthenucleotide(aphosphatelinkedtoasugarlinkedtooneof
fournitrogenousbases);Chargaff’sratios(A=TandG=C);andx-raydiffractionimages(showingthatDNA
isahelixandthemolecule’sdimensions).
Peopledoscience,andmakeintuitiveleapsofgenius,whilealsosometimesmissingtheobviousandmaking
outrightmistakes.AsDr.KarolinLuger,astructuralbiologistandHHMIinvestigatoratColoradoState
University,pointsout,scientistscannotallowthemselvestobeparalyzedbyfearofmakingmistakes,and
althoughmostscientistswanttoprovethattheirhypothesesarecorrect,formallyspeaking,refutinga
hypothesisisalsouseful.PointouttostudentsthatbothPaulingandWatsoninitiallyhypothesizedatriple
helixwiththephosphatesontheinteriorandthebasespointingout,andwhythatwasbasedongoodlogic
thatdidnothoweverfitthedata.Asnewevidenceemerges,modelsaremodifiedandsometimesrejected.
www.BioInteractive.org
RevisedMarch2017
Page11of17
Short Film
The Double Helix
Educator Materials
•
•
ThefilmcontainsmanyillustrationsandanimationsofstructuresofDNAmoleculesandbasepairings.Some
oftheDNAanimationsarebasedonhistoricalmodelsthatarewrong;youmightusetheseanimationsto
testyourstudents’knowledgeofmolecularstructures.IntheanimationinwhichJamesWatsonisbuilding
theDNAmoleculeandthebasesstartcomingtogether,onlytwohydrogenbondsareshownbetweenbases.
TodayweknowthattherearetwohydrogenbondsinanA-TpairandthreehydrogenbondsinaG-Cpair.In
anotheranimationshowingtheDNAmoleculereplicating,bothstrandsarebeingreplicatedinthesame
direction.TodayweknowthatthetwoDNAstrandsarereplicatedinantiparalleldirections,butthe
mechanismofDNAreplicationwasunknownatthetimeofthedouble-helixdiscovery.
Studentsmightbeinterestedinknowingwhothepeoplefeaturedinthefilmare.JamesWatsoniscurrently
atColdSpringHarborLaboratoryinNY;heistheonlyscientistwhowasinvolvedintheoriginalresearch
whowasinterviewedinthisfilm.TheinterviewswithFrancisCrickconsistofhistoricalfootageobtained
whenCrickwasattheSalkInstituteinCalifornia.AfterleavingtheCavendishLaboratory,Watsonjoinedthe
departmentofbiologyatHarvardUniversity,wherehecontributedtoanunderstandingofRNA’sroleinthe
transferofgeneticinformation.HewentontoleadtheColdSpringHarborLaboratoryinNY,wherehe
continuestoconductresearchfocusingonthegeneticbasisofcancerandtowriteaboutscienceinhis
emeritusrole.DuringtheClintonadministration,WatsonspearheadedtheNationalInstitutesofHealth’s
HumanGenomeProject,aneffortthatultimatelyinvolvednumerousscientificleadersandsequencing
centers,aswellashundredsofindividualresearchlabs.Watsonpublishedseveralleadingtextbooks,most
notablyTheMolecularBiologyoftheGene,aswellasnumerouspopularbooksincludingtheclassicscience
discoverystoryTheDoubleHelix.Thefilmnarrator,OliviaJudson,receivedherdoctoratefromOxfordandis
anevolutionarybiologistbasedatImperialCollegeLondon.Sheiswellknownforher2002book,Dr.
Tatiana’sSexAdvicetoAllCreation.Inaddition,shewasfeaturedintheNOVAdocumentary,WhatDarwin
NeverKnew,andwroteaweeklyblogonevolutionarybiologyfortheNewYorkTimeswebsite.Thethree
commentatorsinthefilm,whoprovidedinsightsintothediscoveryanditssignificance,includeSeanB.
Carroll,KarolinLuger,andRobertOlby.SeanB.CarrollisaHowardHughesMedicalInstituteinvestigatorat
theUniversityofWisconsin-MadisonandHHMIvicepresidentforscienceeducation.Carrollisan
internationallyrecognizedevolutionarybiologist.Hisresearchfocusesonthewaynewanimalformshave
evolved.Hisstudiesofawidevarietyofanimalspeciesarerevealinghowchangesinthegenesthatcontrol
animaldevelopmentshapetheevolutionofbodypartsandbodypatterns.Inadditiontohisresearch,
CarrolliswellknownforhisbooksMakingoftheFittest,EndlessFormsMostBeautiful,Remarkable
Creatures,andIntotheJungle.Heisalsoaco-authorofthegeneticstextbookIntroductiontoGenetic
Analysis.Hismostrecentbook,publishedin2013,isentitledBraveGenius.KarolinLugerisanHHMI
investigatoratColoradoStateUniversity.Luger’sresearchinterestsincludethestructuralbiologyof
chromatin,thecomplexofDNAandproteinsthatformschromosomesinthenucleusofeukaryoticcells.In
1997,shedeterminedthestructureofthebasicunitofDNApackaging,orthenucleosome,whichconsistsof
asegmentofDNAwrappedaroundhistoneproteins.Usingthisstructureasastartingpoint,Luger’swork
hasshedlightonhowthenucleosomechangesshape,howchromatininteractswiththecell’stranscription
machinery,andhowsubtlechangesinhistonescanaffectoverallnucleosomestructure.RobertOlbyisa
sciencehistorianattheUniversityofPittsburgh.HeistheauthorofThePathtotheDoubleHelixanda
biographyofFrancisCrickentitledFrancisCrick:HunterofLife’sSecrets.
www.BioInteractive.org
RevisedMarch2017
Page12of17
Short Film
The Double Helix
Educator Materials
RELATEDBIOINTERACTIVERESOURCES
TeacherGuide:DNA(http://www.hhmi.org/biointeractive/teacher-guide-dna)
ThiscurriculumguideassistsinfilteringthroughthevastavailableresourcesfromBioInteractiveandHHMI,and
organizesthematerialaccordingtovarioustopicsrelatedtoDNA,includingDNAstructure.
BuildingBlocksofDNA(http://www.hhmi.org/biointeractive/building-blocks-dna)
Thisanimationshowsthefournitrogenousbases,adenine(A),thymine(T),guanine(G),andcytosine(C),that
makeupDNA.
PairedDNAStrands(http://www.hhmi.org/biointeractive/paired-dna-strands)
ThisanimationshowsthatDNAhasadouble-helixstructure.Ifuntwisted,DNAwouldlookliketwoparallel
strands.EachstrandhasalinearsequenceofA,T,G,andCbases.Thepreciseorderoftheletterscarriesthe
codedinstructions.Onestrandisacomplementaryimageoftheother:AalwayspairswithT,andGalwayspairs
withC.
Chargaff’sRatio(http://www.hhmi.org/biointeractive/chargaffs-ratio)
ThisanimationfeaturesErwinChargaff’s1950paperstatingthatintheDNAofanygivenspecies,theratioof
adeninetothymineisequal,asistheratioofguaninetocytosine.ThisbecameknownasChargaff'sratio,andit
wasanimportantclueforsolvingthestructureofDNA.
Pauling’sTripleHelixModel(http://www.hhmi.org/biointeractive/pauling-triple-helix-model)
ThisanimationshowsLinusPauling'striple-helixmodel,oneofthefailedhypotheticalmodelsofDNA.This
structurewouldbeunstableundernormalcellularconditions.
DNAReplication(http://www.hhmi.org/biointeractive/dna-replication-schematic)
ThestructureofDNA,discoveredbyJamesWatsonandFrancisCrick,suggestsamechanismofreplication.This
animationshowsthatasthedoublehelixunwinds,eachstrandactsasatemplatefortheconstructionofthe
newDNAmolecule.
TheChemicalStructureofDNA(http://www.hhmi.org/biointeractive/chemical-structure-dna)
ThisanimationshowshowDNA'schemicalpropertiescanbeharnessedforavarietyofbiotechnology
applications.
WatsonConstructingBase-PairModels(http://www.hhmi.org/biointeractive/watson-constructing-base-pairmodels)
ThisvideoshowsJimWatsonexplaininghow,duringtheprocessoftryingtoelucidatethestructureofDNA,he
madesomecardboardmodelstounderstandhowDNAnucleotidesarepaired.Themodelshelpedhimvisualize
howhydrogenatomsofpairednucleotidesinteractwitheachothertoformasymmetricalstructurethatfitsthe
double-helixmodel.
USINGTHEQUIZ
Thequizcanhavetwolevels:useonlyquestions1-7forabasiclevelquizandincludequestions8-10formore
depth.Thequizisdesignedasasummativeassessmentthatprobesstudentunderstandingofthekeyconcepts
addressedinthefilm.However,someteachersusethequizbeforeandduringthefilmtoassessstudents’prior
knowledgeandtoguidestudentsastheywatchthefilm.Teachersareencouragedtochoosetheusethatbest
www.BioInteractive.org
RevisedMarch2017
Page13of17
Short Film
The Double Helix
Educator Materials
fitstheirlearningobjectivesandtheirstudents’needs.Teachersareencouragedtomodifythequiz(e.g.onlyask
someofthequestions,explaincomplicatedvocabularyforELLstudents)asneeded.
QUIZQUESTIONSANDANSWERS
1. (KeyConceptF)Inthe1950swhenWatsonandCrickwereworkingontheirmodelofDNA,manyscientists
didnotthinkthatDNAcarriedthegeneticcode.
a. Whatwastheothertypeofmoleculethatsomescientiststhoughtmightcarrygeneticinformation?
Proteins
b. Whydidthisothertypeofmoleculeseemlikealikelycandidate?
Proteinswerefavoredbymanyscientistsbecausetherearemanyproteins,theycomeindifferentshapesand
havemanydifferentfunctions.Also,proteinsaremadeof20differentaminoacidcomponents,whileDNAis
composedofonlyfourbasicsubunits.
2. (KeyConceptA)WhatarethechemicalcomponentsofaDNAnucleotide?
a. aphosphate,asugar,andanitrogenousbase
b. aphosphate,anitrogenousbase,andanaminoacid
c. anitrogenousbase,asugar,andanaminoacid
d. anitrogenousbase,ATP,andasugar
3. (KeyConceptB)ThetwostrandsofaDNAmoleculeareheldtogetherbyhydrogenbondsbetweenthe
a. phosphategroupsoneachstrand
b. nitrogenousbasesoneachstrand
c. basesandthephosphate-sugarbackbone
d. carbonatomsinthesugars
4. (KeyConceptB)Inthediagrambelow,strandsIandIIrepresentthetwocomplementarystrandsofa
portionofaDNAdoublehelix.ThesequenceofstrandIisindicatedbelow.Whatisthesequenceofstrand
II?
StrandI-----------C-T-A-C-----------
StrandII-----------?-?-?-?-------------
a. AGCA
b. CTAC
c. TCGT
d. GATG
5. (KeyConceptC)Theinstructionsforthetraitsofanorganismaredeterminedby
a. theproportionsofA,T,C,andGinDNAmolecules
b. theorderofnucleotidesinDNAmolecules
c. thelengthofDNAmolecules
d. thewaynucleotidesarepairedinthetwostrandsofaDNAmolecule
www.BioInteractive.org
RevisedMarch2017
Page14of17
Short Film
The Double Helix
Educator Materials
6. (KeyConceptsEandF)Scientistsbuildmodelsbasedonwhattheyknowfrompreviousresearchtoderive
testablehypotheses.Independently,bothWatsonandCrickandtheircompetitorLinusPaulingconstructed
anincorrecttriple-helixmodelwiththenitrogenousbasesarrangedsotheywereontheexteriorofthe
moleculeandthephosphategroupsontheinterior.
a. Althoughtheirmodelwaswrong,whatassumptionmadeitreasonabletobuildamodelwiththebases
projecting totheoutside?
Thebasescontainthegeneticinformationastheyvaryinamountbetweenspeciesandintheirarrangement
withinthemolecule;basedonthisinformationitwasreasonabletoassumethatthebaseswouldbeonthe
outsideoftheDNAmolecule.
b. WhatevidencecausedWatsonandCricktorevisetheirmodel?
Basedonthefilm,studentsshouldrealizethatinformationaboutthedimensionsofthedoublehelixfrom
Franklin’simages,aswellasChargaff’spairingrulesforthebases,ledthemtoarevisedmodel.Thefilmsays
thatWatsonhadmisrememberedsomekeymeasurementsfromFranklin’swork.Herimagesshowedthere
wasmuchmorewaterinDNAthanthetriple-helixmodelallowed.Further,thetriple-helixmodelcouldnot
explainhowthethreephosphatebackbonescouldbeheldtogether.(Studentsmaynotknowtheselasttwo
pointsastheyarenotexplicitlystatedinthefilm.)
7. (KeyConceptC)EvenbeforethestructureofDNAwassolved,studiesindicatedthatthegeneticmaterial
musthavethefollowingproperties:
•
beabletostoreinformation;
•
befaithfullyreplicatedandbepassedonfromgenerationtogeneration;and
•
allowforchanges,andthusevolution,tooccur.
ExplainhowthestructureofthedoublehelixshowedthatDNAhadtheseproperties.Writeoneortwo
sentencesperpoint.
Theorderofthebases,A,T,G,andC,containedinformation.BecauseAisalwayspairedwithTandGwithC,
theorderofbasesononestranddeterminestheorderontheotherstrand.Thus,ifaDNAmoleculewere
unwound,eachstrandcouldbecopiedintoacomplementarystrand,producinganexactreplicaofthe
originalmolecule.Errorsinthecopyingmechanismcouldresultinmutations,orchangesintheDNAsequence,
thatcouldbeinheritedbyfuturegenerations.
8. (KeyConceptF)In1928,FrederickGriffithconductedanexperimentinwhichheinjectedmicewithdifferent
kindsofbacteria.Whenbacteriathatcausedisease(pathogenic)wereinjectedinhealthymice,thesemice
gotsickanddied.Othertypesofbacteria(nonpathogenic)didnotcausethemicetodie.Griffithtookthe
DNAfromdeadpathogenicbacteriaandtransferreditintolivingnonpathogenicbacteria.Thesealtered
bacteriaweretheninjectedintohealthymice.Themicediedofthesamediseasecausedbythepathogenic
bacteria.Basedonthisinformation,whichstatementwouldbeavalidconclusion?
a. Whenanorganismdies,theDNAchanges;itnolongerprovidesthesamegeneticinformation.
www.BioInteractive.org
RevisedMarch2017
Page15of17
Short Film
The Double Helix
Educator Materials
b. WhenDNAfromoneorganismistransferredtoanotherorganism,theDNAnolongerfunctions.
c. DNAindifferenttypesofbacteriacarriesexactlythesametypeofinformation.
d. WhenDNAfromoneorganismistransferredtoanotherorganism,itcangivenewtraitstothesecond
organism.
9. (KeyConceptF)ThefollowingtableisasampleofthedataErwinChargaffpublishedin1952.
Proportions*ofNitrogenousBasesintheDNAofDifferentOrganisms
Organism
Tissue
%Adenine
%Guanine %Cytosine
Yeast
31.3
18.7
17.1
Seaurchin Sperm
32.8
17.7
18.4
Rat
Bone
28.6
21.4
21.5
marrow
Human
Thymus
30.9
19.9
19.8
Human
Sperm
30.3
19.5
19.9
%Thymine
32.9
32.1
28.4
29.4
30.3
*Definedasmolesofnitrogenousconstituentsper100gofatomsofphosphate.
Source:E.ChargaffandJ.Davidson,Eds.TheNucleicAcids.AcademicPress,1955.
a. Whichofthefollowingobservationscanbesupportedbythedatainthetable?(Placeacheckmarkinthe
boxnexttothecorrectstatements.)
√Ineachanimal,thepercentageofadenineisthesameasthepercentageofthymineintheDNA.
☐Theproportionsofadenine+thymineandguanine+cytosinearethesameinallorganisms.
☐Largerorganismshavegreateramountsofeachnitrogenousbasethansmallerorganismshave.
☐ThetotallengthofDNAmoleculesinallorganismsisaboutthesame.
b. Inoneortwosentences,explainhowtheseobservationshelpedWatsonandCrickdeveloptheirmodelof
DNA.
Itsuggeststhatinthedoublehelix,adeninealwayspairswiththymineandguaninealwayspairswithcytosine.
c. Inoneortwosentences,explainwhytheproportionsofnitrogenousbasesintheDNAoftwodifferent
humantissues(thymusandsperm)areaboutthesame.
AllbodycellsandtissuesinaparticularorganismcontainthesameDNA.
10. TheimageontherightshowsthefamousphotoB51takeninMay1952byRosalind
FranklinandherstudentRaymondGosling.Thisx-raydiffractionpatternprovided
informationaboutthepositionsofatomsinaDNAmolecule.
a. (KeyConceptG)IdentifytheclueinthisphotothatrevealedthatDNAisahelix.
TheclueistheXshapeintheimage.
www.BioInteractive.org
RevisedMarch2017
Page16of17
Short Film
The Double Helix
Educator Materials
b. (KeyConceptD)Measurementsrevealedthatthedistancebetweenthetwostrandswasalwaysequal.
ExplainhowthisinformationhelpedWatsonandCrickbuildasuccessfulmodelofDNA.
WatsonandCrickconcludedthatalargepurinemolecule(adenineorguanine)alwaysbondswithasmaller
pyrimidinemolecule(cytosineorthymine)—thatway,thedistancebetweenthetwostrandsofDNAisalways
thesame.
c. (KeyConceptE)WasthisinformationconsistentwiththedataobtainedbyChargaff(question9)?Explain
youranswer.
Yes.ChargaffdiscoveredthatintheDNAofanorganism,theproportionofadenine(apurine)isthesameas
theproportionofthymine(apyrimidine).Thesameistrueforguanine(apurine)andcytosine(apyrimidine).
Thesedataareconsistentwithamodelinwhichapurineononestrandalwaysbondswithapyrimidineon
theotherstrand.
KEYREFERENCES
Watson,J.D.,andCrick,F.H.C.1953.Astructurefordeoxyribosenucleicacid.Nature171:737-738.
Wilkins,M.H.F.,Stokes,A.R.,andWilson,H.R.1953.Molecularstructureofdeoxypentosenucleicacids.
Nature171:738-740.
Franklin,R.,andGosling,R.G.1953.Molecularconfigurationinsodiumthymonucleate.Nature171:740-741.
Griffith,F.1928.Thesignificanceofpneumococcaltypes.JournalofHygiene27:113-159.
Avery,O.T.,MacLeod,C.M.,andMcCarty,M.1944.Studiesonthechemicalnatureofthesubstanceinducing
transformationofpneumococcaltypes.TheJournalofExperimentalMedicine79:137-158.
Hershey,A.D.,andChase,M.1952.Independentfunctionsofviralproteinandnucleicacidingrowthofa
bacteriophage.JournalofGeneralPhysiology36:39-56.
Chargaff,E.1950.Chemicalspecificityofnucleicacidsandmechanismoftheirenzymaticdegradation.
Experientia6(6):201-209.
Judson,H.F.TheEighthDayofCreation:MakersoftheRevolutioninBiology.(1979)
Watson,J.TheDoubleHelix:APersonalAccountoftheDiscoveryoftheStructureofDNA.(1968)
Olby,R.ThePathtotheDoubleHelix:TheDiscoveryofDNA(UniversityofWashingtonPress,Seattle:1974&
revised1994).
Olby,R.FrancisCrick:HunterofLife'sSecrets(ColdSpringHarborLaboratoryPress:August25,2009)
AUTHORS
WrittenbyCindyGay,SteamboatSpringsHighSchool,CO,LauraBonetta,PhD,HHMI,andMaryColvard,CobleskillRichmondvilleHighSchool(retired),Deposit,NewYork
EditedbyLauraBonetta,PhD,DennisLiu,PhD,ErikoClements,PhD,HHMIandSusanDodge,consultant;copyeditedby
LindaFelaco;illustrationsbyHeatherMcDonald,PhD
ReviewedbyKarolinLuger,PhD,ColoradoStateUniversity.
FIELDTESTERS
AyseAydemir,LindaCiota,DonnaBalado,JenniferKaltenbach,LauraJulien,LilleenFerraro,EllenPerry,StaceyStrandberg www.BioInteractive.org
RevisedMarch2017
Page17of17